The present invention relates to a printer and a control method thereof.
Printers have various motors such as a paper feed motor for driving a feed roller that conveys punt paper or a print object and a carriage motor for driving a carriage having a print head. DC motors are widely used as such motors to reduce noise. Printers having DC motors are equipped with an encoder composed of a scale having marks or slits disposed at specified intervals and a sensor that senses the marks or slits of the scale to output given signals to control the positions and speeds of the DC motors.
For example, to control a paper feed motor, printers have a disc-shaped scale having multiple slits arranged at specified intervals and a sensor constructed to sandwich each slit between a light-emitting device and a light-receiving device. This type of scale is constructed to rotate with a feed roller. This type of sensor generally outputs two signals with a phase difference of 90° (or example, refer to Japanese Patent Publication No. 2001-232882). The motor is controlled by sensing changing points of the levels of the two signals output from the sensor.
Recently, in order to improve printing quality, an accurate control of a motor mounted on a printer or the like is demanded. In order to perform the more accurate control, signals having high resolution need to be output from an encoder. Here, as a method of outputting signals having high resolution from the encoder, a method which increases a diameter of a disc-shaped scale while keeping the pitches of slits in the related art, or a method which makes the pitches of the slits narrow while keeping the diameter of the scale in the related art is considered.
However, when the diameter of the scale is increased, in case of a printer to be reduced in size, it is difficult to arrange the scale. Further, if the pitches of the slits are made narrow, it is difficult to manufacture the scale.
Further, if the diameter of the scale is increased, a peripheral velocity at the slits is increased. For this reason, if a feed roller rotates at a high speed, high-frequency signals are output from the encoder. Accordingly, a circuit that processes the signals output from the encoder needs to be designed for high-frequency signals, which causes a complex circuit configuration.